1. Technical Field
This invention is a method of producing high-silica substantially alkali-free glasses and high-silica substantially alkali-free glass articles by sintering appropriate field gels below the liquidus.
2. Disclosures of Interest
High silica glasses have wide commercial application, in part, because of their refractory properties, low thermal expansion, high thermal shock resistance, good chemical resistance, and good dielectric and insulating properties when low in alkali-oxides. Recently, high silica glasses have been fabricated with very high purity and such glasses have been successfully applied to produce extremely low-loss optical fibers for communication systems.
High silica glasses are made primarily by either melting or sintering appropriate starting materials. Melting always occurs above the liquidus where the entire body becomes liquid and upon subsequent cooling forms a glass. Sintering generally occurs below the liquidus by viscous flow although the body substantially retaining its shape.
The range of materials available for glass processing by melting is, in one sense, broader than that available for processing by sintering. For example, crystalline solids may be melted to form glass bodies. However, crystalline materials cannot be sintered below the liquidus to a glass body. On the other hand, the formation of silica glass by melting presents significant difficulties in its own right. For example, the high viscosity of molten silica results in a tendency for the molten silica to contain entrapped bubbles which leads to imperfections in the resultant glass. Furthermore, silica must be raised to approximately 2000 degrees to become molten and at that temperature significant evaporation tends to occur. Furthermore, refractory bodies to contain the molten silica at these high temperatures are not readily available.
Applications of glass formation by sintering, well known to those skilled in the art, include sintering milled cullet of previously melted glass, such as in the fabrication of frits, enamels, solder glasses, and slipcast fused silica, the latter particularly useful for electromagnetic windows. Additionally, glass may be formed by sintering particles of amorphous silica which have not been melted, such as, for example, in various optical fiber fabricating techniques, see, for example, U.S. Pat. Nos. 4,217,027 and Re. 28,028.
In U.S. Pat. No. 4,059,658 there is disclosed a technique for forming glass by sintering an appropriately dried gel. The technique, however, requires the use of alkali-containing materials in order to obtain an appropriate gel. Additionally, subsequent leaching steps are used in order to successfully sinter the dried gel without cracking, as well as perhaps to remove undesirable constituents.
In U.S. Pat. No. 4,042,361 there is disclosed a method of densifying a fumed metal oxide which involves drying a flowable sol of metal oxide to form a fragmented solid which is then calcined. The calcined oxide is milled to provide a slip for casting silica articles which are then fused by heating to temperatures above the liquidus.